Obesity among children is a major public health concern in the United States. A recent population-based survey estimated that 10% of children under age 2 are obese (at or above the 95th percentile of the weight-for-recumbent-length growth charts). Although imbalance between energy intake and energy expenditure is certainly a key determinant of weight gain, emerging evidence supports the hypothesis that environmental contaminant exposures, particularly those occurring in early-life, may interfere with homeostatic control and induce or exacerbate obesity. Organophosphate flame retardants (OPFRs) are commonly added to consumer products to reduce flammability and to meet fire safety standards. They have been detected with high frequency in recent studies of home, office, and automobile dust, suggesting that OPFRs leach from products and that widespread human exposure is likely. Although no comprehensive assessment of their distribution in the general population has been undertaken, our preliminary data and several small studies indicate that exposures are ubiquitous. Our proposed research will be the first to assess levels of OPFRs in biological samples from pregnant women and investigate individual characteristics associated with high exposure, identify subgroups at greater risk of high exposure, and assess the potential for adverse health outcomes among exposed offspring. Animal studies provide compelling evidence that OPFR exposures impact growth and development; however, the extent to which these animal studies extend to human populations is unclear. Our primary objective is to investigate the impacts of prenatal OPFR exposure on children's growth from birth through age 3, and in doing so, to fill a critical gap in the understanding of the impacts of OPFRs on human health. We will capitalize on previously collected data and biospecimens from the Pregnancy Infection and Nutrition cohort, a well- characterized cohort with carefully assessed longitudinal measures of growth from birth through age 3. Using stored second trimester urines, we will examine prenatal exposure during the developmental window that is thought to be critically relevant for environmental chemicals that disrupt growth patterning and the development of adiposity. Identifying a link between prenatal OPFR exposure and altered growth in early life could have a substantial public health impact given that even small changes in weight status have been associated with long-term adverse health. Such data is critically needed as use of OPFRs is predicted to increase as other flame retardant compounds are phased-out or banned.